The article addresses the role market definition can play for EU competition practice in the platform economy. The focus is on intermediaries that bring together groups of users whose decisions are interdependent, which therefore are commonly referred to as ‘two-sided platforms’. We address challenges to market definition that accompany these cross-group network effects, assess current practice in a number of competition cases, and provide guidance for adapting practice to properly account for the economic forces shaping markets with two-sided platforms. We ask whether and when a single market can be defined that encompasses both sides. We advocate a multi-markets approach that takes account of cross-market linkages, acknowledges the existence of zero-price markets, and properly accounts for the homing behaviour of market participants.